Magnet assembly having thermal means to adjust the field strength thereof



May 10, 1966 L. MAGNET ASSEMBLY HAVING THERMAL MEANS TO ADJUST THE FIELD STRENGTH THEREOF Filed April 16, 1964 K PARKER 3,250,961

United States Patent 3,250,961 7 MAGNET ASSEMBLY HAVING THERMAL MEANS T0 ADJUST THE FIELD STRENGTH THEREOF Leslie K.-Parker, High Wycombe, England, assignor to Perkin-Elmer Limited, Buckinghamshire, England, a British company Filed Apr. 16, 1964, Ser. No. 360,366 Claims priority, application Great Britain, Apr. 16, 1963, 14,882/ 63 Claims. (Cl. 317-158) This invention relates to permanent magnet assemblies comprising a pair of permanent magnets mounted within a housing forming part of the magnetic circuit. The invention is particularly concerned with such assemblies as are used for nuclear magnetic resonance spectrometers. With such instruments it is important that the magnetic field should be precisely controlled and it is found that variations in the magnetic flux across the working air gap of the magnets are brought about by changes in temperature. Quite apart trom the fact that the magnets themselves are sensitive to temperature, variations in the width of the air gap lead to corresponding variations in its reluctance and hence also in the magnetic flux. These variations arise from the expansion of the mechanical structure of the magnet assembly and it is this expansion which gives rise to the changes in the working air gap.

According to the present invention the magnets are fitted with pole pieces between them defining the Working gap of the assembly. and each pole piece is supported a small distance away from the end of the magnet proper by means of a member supported either by the magnet housing or the respective magnet and which has a coeiiicient of thermal expansion and a dimension in the direction of the axis of the working gap which are so related as to maintain the flux in the working gap substantially constant with variations of temperature.

If the member is supported from the housing its degree of expansion must over-compensate for that of the housing, while if supported from the magnet a degree of compensation is normally obtained since the magnet material has a higher coefficient of linear expansion than that of the housing, but this degree of compensation is not enough to achieve a constant field in the working gap with variation in temperature and the member having yet a higher coefiicient of expansion provides the extra compensation necessary.

Since the magnet material has a negative temperature coeificient, it is necessary, in order to maintain the magnetic field in the working gap constant, to decrease the width of the working gap with an increase of temperature. The member is therefore designed and made of a suitable material so that on experiencing a change of temperature the reluctance across the working gap is so adjusted so as to compensate for change in magnet field strength due to such change in temperature as well as compensating for the differential expansion of the magnet and housing already referred to.

In other words a dual compensation may be achieved to counteract the effects both of thermal expansion and of variation in the magnetic properties of the magnets.

Each support is preferably in the form of an openended tube which is large-r at one end than the other, the smaller end supporting a pole piece and the larger end being supported from the housing from which it is separated by thermal insulation. This has the advantage" that any temperature gradient along the tube is avoided and the temperature of the inside of the tube can be maintained in fixed relationship to that of the magnet. As an alternative a similar form of tubular support may be mounted on the magnet itself so as to experience similar temperatures to those of the magnet.

3,250,961 Patented May 10, 1966 A construction in accordance with the invention will now be described in more detail with reference to the accompanying drawing which is a sectional view through a permanent magnet assembly.

The assembly illustrated comprises a pair of permanent magnets I mounted within a housing formed in two halves 2 and secured together by means of flanges 3. This housing is of magnetic material so as to constitute part of the magnetic circuit of the two magnets. Each magnet 1 has a pole piece 4 and these between them define the working gap of the assembly.

Each pole piece 4 is supported a small distance away from the end of the respective magnet 1 by means of a member 7 of generally tubular shape and which is larger at one end than the other. The small end of the :memher 7 supports the pole piece 4 while the large end is supported from the housing 2 in an adjustable manner, as more specifically described hereinafter.

In a particular example the housing 2 is made of cast iron, the pole pieces 4 of a cobalt-iron alloy such as that known as Permendur and the supporting tubes 7 of aluminum. The magnets 1 are made of a cobalt-nickel-aluminum-copper alloy such as that known as Columax, which has a higher coefiicient of thermal expansion than the cast iron of the housing 2. The aluminum tubes 7 also have a coeflicient of linear expansion greater than that of cast iron which causes the working gap to decrease with increase of temperature. Because the expansion of Columax is .greater than cast iron the secondary air gap between magnet and pole piece also tends to decrease with increase of temperature thus assisting the required commounted by means of screws llsit-uated at intervals of 120 round the housing so as to provide a three-point support. Each screw has a tapered end 12 which acts as a wedge forcing the tube 7 toward the other tube as the screw is screwed inwardly. It all the screws are adjusted by the same amount an axial adjustment of the tube 7 is obtained while individual adjustment of the screws leads to tilting movement of the pole piece 4. To provide for adjustment in a transverse direction secondary screws 13 are also provided at 120 intervals adjacent the pole piece 4 and also require to be adjusted in unison to provide minor adjustments of the pole piece 4.

I claim:

1. In a permanent magnet assembly of the type comprising a pair of permanent magnets mounted within a housing of magnetic material so as to form a main mag- 1 distance away from the end of the magnet proper by being mounted on one part of a supporting member; a different part of said supporting member being attached to said main magnetic assembly; at least a substantial portion of said supporting member between said one part and said difierent part thereof having a coefficient of thermal expansion and a dimension in the direction of the axis of the working gap which are so related as to adjust the axial position of each said pole piece so as to maintain the magnetic field in the working gap substantially constant with variation of temperature.

2. A permanent magnet assembly according to claim 1, in which each supporting member is in the form of an open-ended tube which is larger at one end than the other,

the small end supporting the pole piece and the larger end has provision for transverse adjustment of its supported being supported fmm the housing, pole piece by means of radially extending screws.

3. A permanent magnet assembly according to claim- 1,

in which each supporting member is adjustably mounted References C'ted by the Exammer to permit both axial'movement and tilting of each pole 5 NITED STATES PATENTS piece y m l j m n 3,009,084 11/ 196 1 Balliet 317-177 4. A permanent magnet assembly according to claim 2, 3,182,231 5/1965 Gan-g et al.

in which each tube has three-point adjustment provided by 3,187,237 6/ 1965 Graig et a1.

screw-controlled wedges.

5. A permanent magnet assembly according to claim 4, 1O BERNARD GILHEANY, Primary Examiner.

in which at least one of said tubular supporting members G, HARRIS, J A i nt E a iner, 

1. IN A PERMANENT MAGNET ASSEMBLY OF THE TYPE COMPRISING A PAIR OF PERMANENT MAGNETS MOUNTED WITHIN A HOUSING OF MAGNETIC MATERIAL SO AS TO FORM A MAIN MAGNETIC ASSEMBLY HAVING A COMMON MAGNETIC CIRCUIT, AND POLE PIECES BETWEEN SAID PERMANENT MAGNETS DEFINING THEREBETWEEN THE WORKING GAP OF THE ASSEMBLY; THE IMPROVEMENT IN WHICH EACH POLE PIECE IS SUPPORTED A SMALL DISTANCE AWAY FROM THE END OF THE MAGNET PROPER BY BEING MOUNTED ON ONE PART OF A SUPPORTING MEMBER; A DIFFERENT PART OF SAID SUPPORTING MEMBER BEING ATTACHED TO SAID MAIN MAGNETIC ASSEMBLY; AT LEAST A SUBSTANTIAL PORTION OF SAID SUPPORTING MEMBER BETWEEN SAID ONE PART AND SAID DIFFERENT PART THEREOF HAVING A COEFFICIENT OF THERMAL EXPANSION AND A DIMENSION IN THE DIRECITON OF THE AXIS OF THE WORKING GAP WHICH ARE SO RELATED AS TO ADJUST THE AXIAL POSITION OF EACH SAID POLE PIECE SO AS TO MAINTAIN THE MAGNETIC FIELD IN THE WORKING GAP SUBSTANTIALLY CONSTANT WITH VARIATION OF TEMPERATURE . 